Signal converter circuit



March 22, 1938. D. E. FOSTER 2,111,764

' SIGNAL CONVERTER CIRCUIT Filed April 13, 1956 3 Sheets-Sheet l J1 LOCAL osc/uAro/z ATTORNEY March 22, 1938.

D. E. FOSTER 2,111,764

SIGNAL CONVERTER CIRCUIT V Filed April l3, 1936 3 Sheets-Sheet 2 i TOJIGA/AL /6- SIGNAL l NETWORK sou/e05 LOCAL OJ'C/LLA 70R INVENTOR DUDLEY E. FOSTER ATTORNEY March 22, 1938. DE FOSTER 2,111,764

SIGNAL CONVERTER CIRCUIT Filed April 13, 1956 SSheets-Sheet 3 ZKAMPL 706761144! 0511100014701? SOURCE v I r w LOCAL 0361 8 I TOLENETWORK l ke 70 .SIGA/AL J .SOURCE 1, j

I Q (J INVENTQR LOCAL DUDLEY E. FOSTER 0fC/UA7'0R 2" Y 1 WVL/ AVC AVG ATTORNEY Patented Mar. 22, 1938 PATENT OFFICE SIGNAL CONVERTER CIRCUIT Dudley Foster, Orange, N. .1.,

assignorf to Radio Corporation of America, a corporation of Delaware Application April 13, 1936, Serial no. 73,998

9 Claims.'- (01. 250-20) My present invention relates to radiov frequency signal converter circuits, and more particularly to converters of the electron coupled type.

, Asis well known, the-frequency converter tube,

or first detector, of a superheterodyne receiver p-roducesin its output circuit the sum and difference frequencies of the signal and locally pro,- duced loscillationfrequencies. In addition to these sum and'differencei frequencieathere are also produced-frequencies equal to the sum and difference of integral multiples of the signal and oscillator frequencies. These latter frequencies are usually referred to as higher order effects, since they are due to terms .inthe expression representing the tube characteristic higher. in

order than the term which results in. the desired operating intermediate frequency. For example, in the simple type of converter tube, wherein the signal and oscillator voltages are simultaneously 30 a lied to a common control grid, if. the characteristic of the .plate current be expressed as a power series, i. e., a series whose successive terms are ascending powers of the grid voltage, then the second order (voltage squared) term results 05 in the desired conversion frequencyandhigher orders in undesired frequency terms.

The electron coupled type of frequency converter, and its many circuit forms, is well known at the present time. In such a converter the os- 39 cillator voltage is applied to one control grid whilethe signal voltage is applied-to another grid, the latter being usually spaced fromthe former, and both grids being disposed in the el'ectron stream flowing between the cathode and out-put electrode of the tube. In this type ofconverter the higher order terms give undesired frequencies, just as in the simple type of converter tube. Only those frequencies falling within the response band of theintermediate frequency network'will be 4') transmitted to the second detector, or demodulator. However, any undesired term falling within the intermediate frequency network response band will likewise reach the demodulator, and thus give an undesired response. This difiiculty 3 is most commonly encountered when twice the intermediate frequency falls within the signal frequency range it is desired to receive. In this case, the intermediate frequency can result not only from the difference between the signal and 50 oscillator frequencies, but, also, from the differ-,

ence between twice the signal frequency and the oscillator frequency.

The latter problem can be more clearly understood when viewed in the light of an actual example. Assuming that the operating'I. F. is 500 k. c., thenthe oscillator frequency will be 1500 k. 0. when the receiveris tuned for reception of a 1000 k. 0. signal. .2 If nowv the oscillator frequency be slightlychanged to 1501 k. c., then the result ant I. F. from'the' first order term will be 501 k. c. 5 The secondaorderterm will give a frequency equal to'twicethe' signal frequency, i. e., the difference between 2000 k. c. and 1501 k. c.-, or 499 k. c. It will,.thus, be seen that there will be applied to the second detector. two frequencies of 501 k. c'. 10 andl499- kc c.,:withi the, result that a beat note equal-totheir difference, 2 k. 0., will be heard.

At signal frequencies other than twice the I. F., multiple response will beevidenced instead of audible whistles. Consider again a superheterodyne receiver using a 500 k. c. I. F. with an input signal of 10201:. c. If the oscillator frequency is'1'520 k. 0.," then the 1.1. of 500 k. 0. will be obtained; Furthermore, if the oscillator frequency is 1540 k. .c., the second order term of the signal grid characteristic will give an output of twice signal frequency minus oscillator frequency, or

2040 k. 0., minus 1540 k. c., resulting in 500 k. c.

It is seen, therefore, that for any applied signal frequency, there-are at least two oscillator frequencies which willresult in the correct I. F.

Accordingly, it may be stated that it is one of the main. objects of my present invention substantially to reduce undesired responses in a converter of the electron coupled type, which re v sponses correspond to terms higher than the first order term in the expression representing the converter tube characteristic; the reduction in undesired responses being accomplished by applying the local oscillator voltage simultaneously to-the signalrand oscillator grids, but the oscillator voltage being impressed on the two grids in out-of-phase relation, and the magnitude of the out-of-phase oscillator voltage being chosen to secure the substantialreduction of the higher orderterms referred to.

Anotherimportant object of the present invention is to cancel from the output circuit of an electron coupledtype of frequency converter, undesired responses produced by the second order effect; and this-cancellation being accomplished by applying thelocal oscillator voltage simultaneously to the oscillator grid and the signal grid, the oscillator voltage being applied in reversed phase to the respective grids; it being pointed out that since the second order effect is less than the first order effect, the amount of local oscillator voltage on the signal grid which causes cancellation of the second order effect will result in only a slight decrease in the desired first order frequency conversion.

In existing electron discharge tubes of the type employed in electron coupled converters, the amount of local oscillator voltage to be applied to the signal grid for cancellation of undesired responses, is a function of the bias on the signal grid since the tube characteristic with respect to the signal grid is not purely exponential. Hence, it may be stated that it is another object of the present invention to apply local oscillator voltage of an electron coupled converter to the signal and oscillator grids in phase and magnitude relations substantially to reduce the higher order effects resulting in undesired responses in thefrequency range it is desired to receive, and yet employ automatic volume control action on the converter; this dual result being secured by utilizing a phase inverter tube to impress local oscillator voltage on the signal grid, and the bias of the inverter tube being varied concurrently with that of the signal grid of the signal converter tube with the result that the'proper amount of local oscillator voltage is :applied to the signal grid for any value of signal grid bias.

Another object of the invention may be stated to reside in the provision of an electron coupled converter tube which not only has applied-to its signal grid a local oscillator voltage in phase and magnitude substantially to eliminate the-undesired responses due to the higher order efiects, but wherein the direct current voltageof the oscillator and signal grids are varied in dependence upon received signal amplitude variation.

Still other objects of the present invention are to improve generally the efliciency and operating reliability of frequency converters of the electron coupled type, and more especially to provide such converters which are not only durable, reliable and simple in operation, but economically manufactured and assembled in radio receivers of the superheterodyne type.

The novel features which I believeto be characteristic of my invention are set forth in particularity in the appended claims; thev invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings:

Fig. 1 is a schematic circuit diagram showing the basic circuit employed in this invention,

Fig. 2 shows a converter network embodying one form of the invention,

Fig. 3 shows an alternative arrangement of the invention, 1

Fig. 4 illustrates another embodiment/of the invention,

Fig. 5 illustrates another converter arrangement, 1

Fig. 6 shows a converter network embodying still another form of the invention,

Fig. 7 shows a converter network employing the invention, and utilizing automatic volume control,

Fig. 8 shows a modification of the arrangement of Fig. 7.

Referring now to the accompanying drawings, wherein like reference charactersin the difierent figures designate similar circuit elements, there is shown in Fig. 1 a purely schematicrepresentation of the basic principle involved in the present invention. The numeral l designates a pentagrid converter tube which is well known to those skilled in the art, and is usually designated as a 2A7. The input electrodes of the tube are connected across a tuned signal input circuit 2, the latter circuit being coupled to a source of signal energy. The local oscillator is schematically represented by the numeral 3, and is connected between the first grid and the grounded side of the cathode bias resistor 4, the latter being shunted by the usual by-pass condenser 5. The fourth grid 6 is connected to the high alternating potential side of the input circuit 2, and the signal grid 6 and oscillator grid 1 have disposed between them a pair of positively biased grids. A positively biased grid is also disposed between the output plate electrode 8 and the signal grid 6.

The numeral 9 denotes the usual variable tun- :ing condenser disposed in the input circuit 2,

and the condenser 9 is to be understood as being adjustable over a relatively wide frequency range, as for example the broadcast range of 500 to 1500 k. 0. Of course, the tuning range of circuit 2 may be in the higher frequencies, and it is to be clearly understood that the local oscillator circuit 3 is simultaneously tunable, by means of its own tuning element, over a frequency range which differs from the signal frequency range by the operating I. F. The I. F. network is disposed in the circuit connected to the plate 8.

It is not believed necessary to go into the details of the functioning of the converter tube I. Those skilled in the art are fully aware of the fact that there is produced in the circuit connected to the outputelectrode 8, the sum and difference frequencies between the applied signal frequency and the local oscillator frequency, as well as frequencies equal to the sum and difference of integral multiples of the signal and oscillator frequencies. The electron stream flowing between the cathode and plate 8 is modulated at the frequencies of the signal and oscillator voltages, and the various modulation frequencies, as explained above, are produced in the circuit connected to the plate 8. The I. F. network being resonated to the desired operating I. F., only those frequencies falling within the response band of the I. F. network will be transmitted to the demodulator.

However, undesired responses produced by the second, third or higher order effects will likewise reach the demodulator, and give undesired responses if any undesired term falls within the I. F. network response band. To reduce substantially these undesired responses there is impressed upon the signal grid 6 a portion of the locally produced oscillator voltage, and this impression is made through the impedance Z. The impedance Z is designed to provide the proper phase so as to produce reduction of the higher order effects.

The impedance Z may be inductive or capacitative; the various following figures illustrate the impedance in these two aspects. The impedance Z must be chosen of such phase and magnitude as to cancel out the response due to the second order term. If the third, or higher, order effect results in undesired responses in the desired frequency-range, then the amount of out-of-phase oscillator voltage applied to the signal grid 6, through impedance Z, isadjusted to cancel higher order responses.

In Fig. 2, there is shown a converter network wherein the out-of-phase oscillator voltage is applied to the signal grid 6 of the converter tube l (of the 2A7 type) through condenser ID. "The latter is made adjustable so that it can be varied in magnitude to the point where the undesired responses in I. F. network I I can be cancelled. The condenser I0 is connected between the grid 6 and the high alternating potential side of the tunable oscillation circuit 2' of the local oscil'-' lator. The dotted line l2 denotes the usual mechanical coupling between the rotors of the variable tuner condensers 9 and 9'. It is to be clearly understood that the coupling between the oscillator grid 7 and the oscillator is conventional. Further, the tuner circuits may be of the multirange type, if desired.

The circuit arrangement shown in Fig. 3 embodies a form of the invention wherein the signal grid 6, disposed adjacent the cathode of the converter tube l, is connected to the plate of the oscillator tube 3 through condenser In. The oscillator is of a conventional form, and needs no detailed description. The oscillator grid 1, dis posed in the positive shielding field, is connected to the grid of the oscillator tube. The condenser It] may beadjusted to balance out the second harmonic effects in the I. F. output network II.

The circuit arrangement of Fig. 4 diifers from the preceding arrangement in the harmonic cancellation coupling between the local oscillator and the signal grid 6. This coupling comprises the mutual inductance M provided between coil l3 and tunable circuit coil 14. The magnitude of M may be varied until the undesired second harmonic response in network II is cancelled out.

With the oscillator higher in frequency than the signal, the signal circuit appears capacitive to the oscillator frequency. If the signal circuit were a pure capacity, then the feedback coupling and signal tank circuit would act as a simple capacitive voltage divider. circuit appears as capacity and resistance, so for phase as well as magnitude balance the feedback circuit should comprise resistance, which may be in series or parallel with the feedback capacity. In practice, appreciable reduction is secured without phase balance, but better reduction of spurious components will result from phase as well as magnitude adjustment. Similarly a resistance in conjunction with M would produce better balance by adjustment of phase as well as magnitude. Capacitive and inductive couplings are shown to indicate alternative arrangements.

The modification shown in Fig. 5 involves the connection of the signal gridB to the high alternating potential side of signal input circuit 2. The local oscillations are impressed'on the signal grid 6 by connecting the low alternating potential side of oscillation circuit coil l5 to the low alternating potential side of signal circuit coil [4 through adjustable condenser ill. The junction of condenser H) and coil I4 is grounded through condenser U5. The oscillator grid 1 is connected to the high alternating potential side of oscillator circuit 2. Coil l5 of Fig. 5, padding cone densers iii and iii and the variable condensers connected to coil l5 represent the tank circuit of a separate oscillator tube. The common coupling to the signal circuit is condenser l5. Condenser l6 is the cancellation coupling condenser; condenser It is added in series therewith, the series combination of condensers Hi and Ill acting as an oscillator padding condenser. Adjustment of It affects padding, but does not affect the coupling magnitude to signal network.

However, the tank.

In the circuit arrangement of Fig. 6, the converter tube I is of the type shown in Fig. 2.

- menta decrease of approximately 20 db. in the second order effect was obtained upon proper proportioning of M1.

When AVC action is appliedto the converter tube the second harmonic cancellation is afiected. This arises by virtue of the fact that the signal grid characteristic is not purely exponential in existing tubes. The amount of oscillator voltage to be applied to the signal grid for harmonic cancellation is a function of the bias on the signal grid. Since the signal grid bias varies in magnitude when AVC is used, it follows that the'harmonic cancellation voltage will also vary. It, therefore, becomes necessary to employ a device for insuring the application of the proper amount of oscillator voltage for any signal grid bias.

Fig. '7 shows one arrangement for automatically correlating the harmonic cancellation voltage with the converter signal bias value. The local oscillator applies the oscillations to the oscillator grid 1 through condenser C2. The AVC network 20, of conventional type, automatically regulates the conversion gain of tube l. The AVC network, in general, may embody a signal rectifier supplied with I. F. energy, and which rectifier furnishes the direct current voltage component for automatically regulating the signal grid bias of the converter tube I. Of course, the AVG network may also regulate any of the high frequency signal transmission tubes in gain. Furthermore, the AVG rectifier maybe an independent rectifier, or may be part of the demodulator of the system, and in the latter case the direct current voltage component of detected I. F. voltage would be used for the AVG action.

The harmonic cancellation oscillator voltage is applied to the signal grid .5 through a phase inverter tube 2!. The electron discharge tube 2| has its plate connected to the high alternating potential side of signal circuit 2 through a condenser C3, the plate being supplied with proper positive voltage through the choke 22. The control grid of the inverter tube 2! is connected to the high alternating potential side of the local oscillator circuit 2' through condenser C4, and the cathode of the inverter tube is grounded through bias network 24.

When bias increases on tube l, the bias decreases on tube 2|. By taking screen voltage for tube i through resistors 25 and 26, as tube l is biased negatively the screen potential increases in a positive direction, and with it, to a lesser degree, the potential of point 2? of resistor 26. By connecting the control grid of tube 2|, through resistor 23, to point 27, the grid potential of tube 2! becomes less negative'when that of tube I becomes more negative. Resistor 24 in the cathode of 2! is required so that the cathode of tube 2| is at greater positive potential, relative to ground, than point 21; the grid of tube 2| thus being maintained negative with respect to its own cathode.

In this way a change in gain control bias on the signal grid 6 will automatically vary the bias on the grid of the inverter tube 2!, and the gain of the latter will be varied. The function of the tube 2| is to invert the phase of the applied local oscillator voltage so as to secure the proper harmonic cancellation phase. The variation in gain of tube 2| is chosen so that the proper amount of oscillation voltage is applied for every change in converter signal grid bias. In this way the departure of the converter signal grid characteristic from the pure exponential type is compensated for; the compensation involves the automatic correlation of the magnitude of the applied harmonic cancellation oscillator voltage with the value of the converter signal grid bias.

At the expense of some conversion sensitivity the phase inverter tube 2! may be dispensed with. The local oscillator, conventionally represented in Fig. 8, is shown coupled, through condensers, to the local oscillator grid T and the signal grid 6. The AVC connections, from the usual AVC network, are made to the signal and oscillator grids. The bias of the oscillator grid is auto-- matically varied with that of the signal grid in this modification. This requires that the oscillator grid I draw little current for practical AVC action, and it must, therefore, be run negative. Both grids vary in the same polarity sense, but at different rates. In general, grid 6 varies less than grid 1.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In an electron discharge tube which is provided with at least a cathode, an output electrode and an electron path therebetween, the method which includes drawing electrons to an intermediate point in said path, retarding said electrons to form a virtual cathode which is beyond said point, modulating the density of the virtual cathode by varying the attraction of said electrons through attracting the part of said electrons from said virtual cathode to said output electrode, causing a received signal to vary the further attraction of said part of the electrons,

- the modulation of the density of the virtual cathode thereby causing modulation of the signal in said tube, and simultaneously modulating said part of the electrons in out-of-phase relation with said first modulation and to an extent such as to reduce substantially the appearance of second order, or higher, term effects.

2. In a superheterodyne receiver, a first detector system comprising a tube having a cathode and an oscillation electrode near said cathode, means electrically connected with said oscillation electrode for varying the alternating current voltage thereof at a predetermined locally produced oscillation frequency, said tube including a control grid and an anode located in the space path beyond said oscillation electrode, a source of signal voltage coupled between said control grid and said cathode, and additional means connected between said control grid and said first means for impressing upon said control grid said 10- cally produced oscillations in phase and magnitude such that undesired responses due to the second order, or higher, term of the detection characteristic are substantially reduced.

3. In a superheterodyne receiver, a first detector system comprising a tube having a cathode and an oscillation electrode, means electrically connected with said oscillation electrode for varying the alternating current voltage thereof at a predetermined locally produced oscillation fre quency, said tube including a control grid and an anode, said control grid being located in the space path between the cathode and said oscillation electrode, a source of signal voltage coupled between said control grid and said cathode, and additional means connected between said control grid and said first means for impressing upon said control grid said locally produced oscillations in phase and magnitude such that undesired responses due to the second order, or higher, term of the detection characteristic are substantially reduced, means for developing a bias control voltage which automatically varies with the intensity of said signal source, means for applying said bias voltage to said control grid, and additional means, responsive to the variation in .said bias control voltage, for automatically correlating the local oscillation voltage impressed on said control grid with the change in said bias control voltage.

4. In combination in a first detector network of a superheterodyne receiver, an electron discharge tube provided with at least a cathode, an output electrode, a pair of control electrodes disposed between the cathode and output electrode, and means electrostatically shielding said pair of control electrodes, a tuned signal input circuit connected between the cathode and one of the control electrodes, means coupled to the cathode and the other control electrode for varying the alternating current voltage of said other control electrode at a frequency rate which difiers from the signal frequency by a predetermined intermediate frequency, and means coupling said input circuit and said varying means for impressing on said first control electrode alternating current voltage varying in frequency at said oscillation frequency rate, the phase of the alternating current voltage impressed on said first control electrode being related to the phase of the voltage on said other control electrode substantially to reduce the efiect of the second order term of the detection characteristic on the electron stream flowing between the cathode and said output electrode.

5. In a first detector network of a superheterodyne receiver, an electron discharge tube which includes a cathode, an anode and at least two control grids disposed in the electron stream between the cathode and anode, a resonant network in the anode circuit which is tuned to an operating intermediate frequency, a tunable signal input circuit connected between the cathode and one of the control grids, a source of local oscillations, variable over a predetermined local oscillator frequency range, coupled between the cathode and the other control grid, and means, electrically connected with said local oscillation source and the said signal input circuit, for impressing upon said first control grid local scillator voltage which is out-of-phaserwith the oscillator voltage impressed on the other control grid.

6. In a first detector network of a superheterodyne receiver, an electron discharge tube which includes a cathode, an anode and at least two control grids disposed in the electron stream between the cathode and anode, a resonant network in the anode circuit which is tunedto an operating intennediatefrequency, a tunable signal input circuit connected between the cathode and one of the control grids, a source of local oscillations, variable over a predetermined local oscillator frequency range, coupled between the cathode and the other control grid, and means, electrically connected with said local oscillation source and the said signal input circuit, for impressing upon said first control grid local oscillator voltage which is out-of-phase with the oscillator voltage impressed on the other control grid, means, responsive to variations in signal amplitude, for automatically regulating the first control grid bias, and additional means responsive to said last named means for varying the magnitude of the oscillator voltage impressed on said first control grid.

'7. A frequency converter tube comprising a cathode, an anode and at least two additional electrodes disposed in succession in the electron stream between the cathode and anode, a source of oscillations connected to one of the said electrodes to vary the potential thereof at a predetermined frequency, a source of signals connected to the other electrode to vary the voltage thereof at a frequency different from the oscillation frequency, a circuit connected to the anode to utilize the frequency component of anode current equal to the difference between said signal and oscillation frequencies, and a high frequency coupling path, independent of said electron stream, between said electrodes, said path including a reactive impedance whose magnitude and phase are chosen so that said oscillations are impressed on said signal electrode in out-of-phase relation with the oscillations impressed on said one electrode thereby to prevent the production of harmonic responses in said anode circuit.

8. A frequency converter tube comprising a cathode, an anode and at least two additional electrodes disposed in succession in the electron stream between the cathode and anode, a source of oscillations connected to one of the said electrodes to vary the potential thereof at a predetermined frequency, a source of signals connected to the other electrode to vary the voltage thereof at a frequency different from the oscillation frequency, a circuit connected to the anode to utilize the frequency component of anode current equal to the difference between said signal and oscillation frequencies, a high frequency coupling path, independent of said electron stream, between said electrodes, said path including a reactive impedance whose magnitude and phase are chosen so that said oscillations are impressed on said signal electrode in out-of-phase relation with the oscillations impressed on said one electrode thereby to prevent the production of harmonic responses in said anode circuit, and means, responsive to signal amplitude changes, to vary the gain of said converter tube.

9. In a first detector network of a superheterodyne receiver, anelectron discharge tube which includes a cathode, an anode and at least two control grids disposed in the electron stream between the cathode and anode, a resonant network in the anode circuit which is tuned to an operating intermediate frequency, a tunable signal input circuit connected between the cathode and one of the control grids, a source of local oscillations, variable over a predetermined local oscillator frequency range, coupled between the cathode and the other control grid, and means electrically connected with said local oscillation sourceand the said signal input circuit, for impressing upon said first control grid local os- 

